A bit more info on the dialog line: Vergil is trying to warn the player about the cop ally.

When you have 29 audio logs, the cop isn't killed by drones and later leads the player to the body of Sadie's father. He explains his job was to make sure her father was dead and that no one knows, including the player, who he then tries to kill.

What's interesting to me about that is that the cop is the first (and only) human enemy you encounter in any Halo game (except, of course, betraying your allies and whatnot)